ACTION OF LEAVES. 53 



ference of such species seems to consist chiefly in the 

 epidermis, which regulates the amount of perspi- 

 ration (61). It is therefore to be remarked, that it is 

 not the greatest quantity of light which can be 

 obtained that is most favourable to the healthiness of 

 plants, but the greatest quantity they will bear with- 

 out injury. If the former were true, the concentrated 

 light of a lens would be better than the strongest 

 ordinary light; but the effect of the concentrated 

 light of a lens is to burn the surface, and the ordi- 

 nary solar rays produce the same effect upon many 

 plants, probably by exhausting the tissue of its water 

 faster than it can be supplied from the roots. 



77. In the course of time, a leaf becomes incapable 

 of performing its functions ; its passages are choked 

 up by the deposit of sedimentary matter ; there is no 

 longer a free communication between its parenchyma 

 and that of the rind, or between its veins and the 

 wood and liber. It changes colour, ceases to decom- 

 pose carbonic acid, absorbs oxygen instead, gets into 

 a morbid condition, and dies : it is then thrown off. 

 This phenomenon, which we call the fall of the leaf, is 

 going on the whole year round, except mid-winter, 

 in some plant or other. Those which lose the whole 

 of their leaves at the approach of winter, and are 

 called deciduous, begin, in fact, to cast their leaves 

 within a few weeks after the commencement of their 

 vernal growth ; but the mass of their foliage is not 

 rejected till late in the season. Those, on the other 

 hand, which are named evergreens, part with their 

 leaves much more slowly; retain them in health 



